The present invention relates to a drop-on-demand ink-jet printing apparatus, and more particularly, to an ink-jet printing head in which a droplet of printing fluid is ejected from a nozzle by a volume displacement thereof.
A well known ink-jet printing apparatus prints a desired pattern on a recording medium, such as paper, by depositing discrete droplets of printing fluid (ink) on the recording medium. One such ink-jet printing apparatus is disclosed in U.S. Pat. No. 4,189,734 issued to Kyser et al. Kyser et al. teach a structure of a printing head which includes a deflection plate bonded to a base plate to form a chamber. The chamber is filled with the ink and is provided with a nozzle at one end. A piezoelectric transducer is bonded to the deflection plate and connected to an electronic driver circuit. Upon application of voltage across the piezoelectric transducer, the transducer contracts to cause the deflection plate to deflect inwardly into the chamber. Thus, the volume of the chamber is reduced, causing a droplet of the printing fluid to be ejected from the orifice of the nozzle.
In the conventional apparatus, the piezoelectric transducer is fixed on the deflection plate at a position which is unrelated to the pressure vibration modes of the ink in the chamber. Accordingly, the piezoelectric transducer generates a pressure vibration wave combining a plurality of the pressure vibration modes.
In order to print Chinese characters, half-tone images, and the like with a high printing resolution, an ink-jet printing apparatus is required to generate a fine droplet of ink, i.e. to reduce the volume of the droplet. In general, the droplet volume Q is related to the sectional area A of the nozzle, the droplet velocity v(t) at the orifice of the nozzle, the time t.sub.1 when the pressure of the piezoelectric transducer is applied to the ink in the chamber, and the time t.sub.2 when the droplet of the ink is separated from the orifice of the nozzle, as follows: ##EQU1## The droplet velocity v(t) is proportional to the voltage applied to the piezoelectric transducer. The period of time from t.sub.1 to t.sub.2 is determined by the configuration of the chamber and the disposition of the piezoelectric transducer with respect to the chamber.
According to formula (1), the droplet volume Q is reduced if the sectional area A of the nozzle is decreased. However, it is difficult to manufacture a fine nozzle, and a fine nozzle is apt to be plugged with ink. Another way to reduce the droplet volume Q is to decrease the droplet velocity v(t), i.e., to decrease the voltage applied to the piezoelectric transducer. However, a low speed droplet is difficult to project accurately due to the deflection of its trajectory. Accordingly, a fine ink droplet is difficult to obtain in the conventional ink-jet printing head.